The present invention relates to a low noise ball valve assembly and, more particularly, to such an assembly for controlling the transmission and distribution of a fluid.
In the transmission and distribution of compressible fluids, such as natural gas, there are requirements for valves that control a variable, such as pressure or flow rate, and operate at high pressure drops, that is, high pressure differentials between the upstream and downstream pressure. As such, these valves are fitted with actuators and positioners that respond to a control signal generated by a controller or computer.
When a compressible fluid is throttled through a control valve at a high pressure drop, no is generated in the fluid aerodynamically, and subsequently is propagated through the fluid, exciting the pipe walls (principally downstream), thereby causing noise to be propagated to the surrounding atmosphere. The result may be noise that exceeds allowable limits for worker hearing conservation.
A second concern involved with the throttling of a compressible fluid through a control valve is that it often causes excessive mechanical vibration results in attendant problems with the proper operation of associated measuring and controlling equipment. In addition, the vibration can also cause fatigue failure of welds or piping.
Ball valves are frequently used for shutoff valves and for control valves for special applications, such as the transmission and distribution of natural gas. In order to reduce noise and mechanical vibration when ball valves are used, inserts have been placed in the ball valves which are provided with a plurality of relatively small-diameter passages through which the fluid passes under certain flow conditions. However, the availability of inserts for ball valves that offer significant reduction of noise and mechanical vibration have been very limited.
Also, ball valves of the above type are often limited to applications in which there is a high pressure drop throughout the entire range of travel of the valve. In these cases, the valves are designed for the continuous reduction of noise and mechanical vibration over their entire range of travel. However, there are applications that involve a relatively high pressure drop at relatively low flow rates and small valve openings, and a relatively low pressure drop at maximum flow and relatively large valve openings. In the latter, low pressure-drop situation, a flow capacity is required that is higher than would be possible utilizing a valve designed for continuous noise reduction based on a high pressure drop throughout the entire valve travel range.
Also, ball valves that have inserts of the above type that are welded, or otherwise attached, within a spherical ball, are difficult to manufacture and often cause distortion of the ball valve. Further, ball valves having inserts of the above type can cause separation of the gas flow as it passes through the valve which results in losses in a pressure drop which compromises the performance of the valve.
Therefore what is needed is a ball valve that can reduce noise at relatively low flow rates and small valve openings at relatively high pressure drops, yet can respond to relatively low pressure drop situations and achieve maximum flow. Also needed is a ball valve of the above type that reduces mechanical vibration, is relative easy to manufacture, and is not easily subjected to distortion. Further, a ball valve of the above type is needed that eliminates flow separation of the gas as it passes through the valve.